The kinetics of adenyl cyclase have been investigated in the hepatocyte, the adrenal, and the fat cell. The mechanism of catalysis has been described as well as the mode of regulation by nucleotides and hormones. Sophisticated techniques of simulation and analysis are used to construct experiments yielding optimal kinetic information about the enzyme. Adenyl cyclase has now been shown to possess three intrinsic states of catalytic activity showing marked differences in several parameters of catalysis. Guanine nucleotides and hormones of various types have been shown to act on adenyl cyclase by modulating the distribution among these states. Both steady state and transient kinetics have been successfully analyzed.